The invention relates to the field of thrusters. Thrusters are used for propelling spacecrafts, with a typical exhaust velocity ranging from 2 km/s to more than 50 km/s, and density of thrust below or around 1 N/m2. In the absence of any material on which the thruster could push or lean, thrusters rely on the ejection of part of the mass of the spacecraft. The ejection speed is a key factor for assessing the efficiency of a thruster, and should typically be maximized.
Various solutions were proposed for spatial thrusters. U.S. Pat. No. 5,241,244 discloses a so-called ionic grid thruster. In this device, the propelling gas is first ionized, and the resulting ions are accelerated by a static electromagnetic field created between grids. The accelerated ions are neutralized with a flow of electrons. For ionizing the propelling gas, this document suggests using simultaneously a magnetic conditioning and confinement field and an electromagnetic field at the ECR (electron cyclotron resonance) frequency of the magnetic field. A similar thruster is disclosed in FR-A-2 799 576, induction being used for ionizing the gas. This type of thruster has an ejection speed of some 30 km/s, and a density of thrust of less than 1N/m2 for an electrical power of 2,5 kW. One of the problems of this type of device is the need for a very high voltage between the accelerating grids. Another problem is the erosion of the grids due to the impact of ions. Last, neutralizers and grids are generally very sensitive devices.
U.S. Pat. No. 5,581,155 discloses a Hall Effect Thruster. This thruster also uses an electromagnetic field for accelerating positively-charged particles. The ejection speed in this type of thruster is around 15 km/s, with a density of thrust of less than 5 N/m2 for a power of 1,3 kW. Like in ionic grid thruster, there is a problem of erosion and the presence of neutralizer makes the thruster prone to failures.
U.S. Pat. No. 6,205,769 or D. J. Sullivan et al., Development of a microwave resonant cavity electrothermal thruster prototype, IEPC 1993, no36, pp. 337-354 discuss microwave electrothermal thrusters. These thrusters rely on the heating of the propelling gas by a microwave field. The heated gas is ejected through a nozzle to produce thrust. This type of thruster has an ejection speed of some 9-12 km/s, and a thrust from 200 to 2000 N.
D. A. Kaufman et al., Plume characteristic of an ECR plasma thruster, IEPC 1993no37, pp. 355-360 and H. Tabara et al., Performance characteristic of a space plasma simulator using an electron cyclotron resonance plasma accelerator and its application to material and plasma interaction research, IEPC 1997no 163, pp. 994-1000 discuss ECR plasma thrusters. In such a thruster, a plasma is created using electron cyclotron resonance in a magnetic nozzle. The electrons are accelerated axially by the magnetic dipole moment force, creating an electric field that accelerates the ions and produces thrust. In other words, the plasma flows naturally along the field lines of the decreasing magnetic field. This type of thruster has an ejection speed up to 35 km/s. U.S. Pat. No. 6,293,090 discusses a RF plasma thruster; its works according to the same principle, with the main difference that the plasma is created by a lower hybrid wave, instead of using an ECR field.
U.S. Pat. No. 6,334,302, U.S. Pat. No. 4,893,470 or Dr. Franklin R. Chang-Diaz, Design characteristic of the variable Isp plasma rocket, IEPC 1991, no 128, disclose variable specific impulse magnetoplasma thruster (in short VaSIMR). This thruster uses a three stage process of plasma injection, heating and controlled exhaust in a magnetic tandem mirror configuration. The source of plasma is a helicon generator or MagnetoPlasmaDynamic (MPD) Thruster and the plasma heater is a cyclotron generator working at Ion Cyclotron Frequency. The “hybrid plume”, composed of hot plasma core surrounded by cold gas is contained in a nozzle which is protected from the hot plasma by the cold gas blanket. This thermal expansion in a nozzle converts a part of the internal energy into directed thrust. As in ECR or RF plasma thruster, ionized particles are not accelerated, but initially flow along the lines of the decreasing magnetic field and then along the gradient of pressure. This type of thruster has an ejection speed of some 10 to 300 km/s, and a thrust of 50 to 1000 N.
In a different field, U.S. Pat. No. 4,641,060 and U.S. Pat. No. 5,442,185 discuss ECR plasma generators, which are used for vacuum pumping or for ion implantation. Another example of a similar plasma generator is given in U.S. Pat. No. 3,160,566.
U.S. Pat. No. 3,571,734 discusses a method and a device for accelerating particles. The purpose is to create a beam of particles for fusion reactions. Gas is injected into a cylindrical resonant cavity submitted to superimposed axial and radial magnetic fields. An electromagnetic field at the ECR frequency is applied for ionizing the gas. The intensity of magnetic field decreases along the axis of the cavity, so that ionized particles flow along this axis. This accelerating device is also discloses in the Compte Rendu de I'Academie des Sciences, Nov. 4, 1963, vol. 257, p. 2804-2807. The purpose of these devices is to create a beam of particles for fusion reactions: thus, the ejection speed is around 60 km/s, but the density of thrust is very low, typically below 1,5 N/m2.
U.S. Pat. No. 3,425,902 discloses a device for producing and confining ionized gases. The magnetic field is maximum at both ends of the chamber where the gases are ionized.
Thus, there is a need for a thruster, having a good ejection speed, which could be easily manufactured, be robust and resistant to failures. This defines an electrode-less device accelerating both particles to high speed by applications of a directed body force.
The invention therefore provides, in one embodiment a thruster, having
a chamber defining an axis of thrust;
an injector adapted to inject ionizable gas within the chamber;
a magnetic field generator adapted to generate a magnetic field, said magnetic field having at least a maximum along the axis;
an electromagnetic field generator adapted to generate                a microwave ionizing field in the chamber (6), on one side of said maximum; and        a magnetized ponderomotive accelerating field on the other side of said maximum.        
The thruster may also present one or more of the following features:
the angle of the magnetic field with the axis is less than 45°, preferably less than 20°;
the frequency of the electromagnetic field is within 10% of the electron cyclotron resonance frequency at the location where the electromagnetic field is generated;
the ratio of the maximum value to the minimum value of the magnetic field is between 1,1 and 20;
the angle of the electric component of the electromagnetic field with the orthoradial direction is less than 45°, preferably less than 20°;
the local angle between the electric component of electromagnetic field and the magnetic field in the thruster is between 60 and 90°;
the ion cyclotron resonance period in the thruster is at least twice higher than the characteristic collision time of the ions in the thrusters;
the microwave ionizing field and the magnetic field are adapted to ionize at least 50% of the gas injected in the chamber;
the magnetic field generator comprises at least one coil located along the axis substantially at the maximum of magnetic field;
the magnetic field generator comprises a second coil located between said at least one coil and said injector;
the magnetic field generator is adapted to vary the value of said maximum;
the magnetic field generator is adapted to vary the direction of said magnetic field, at least on said other side of said maximum;
the electromagnetic field generator comprises at least one resonant cavity;
the electromagnetic field generator comprises at least one resonant cavity on said one side of said maximum;
the electromagnetic field generator comprises at least one resonant cavity on said other side of said maximum;
the chamber is formed within a tube;
the tube has an increased section at its end opposite the injector;
the thrusters comprises a quieting chamber between the injector and the chamber.
The invention further provides a process for generating thrust, comprising:
injecting a gas within a chamber;
applying a first magnetic field and a first electromagnetic field for ionizing at least part of the gas;
subsequently applying to the gas a second magnetic field and a second electromagnetic field for accelerating the partly ionized gas due to the magnetized ponderomotive force.
The process may further be characterized by one of the following features:
the gas is ionized by electron cyclotron resonance and accelerated by magnetized ponderomotive force;
the ions are mostly insensitive to the first magnetic field;
the local angle between the first electric component of electromagnetic field and the first magnetic field is between 60 and 90°;
the local angle between the electric component of second electromagnetic field and the second magnetic field is between 60 and 90°;
at least 50% of the gas is ionized;
the direction of the second magnetic field is varied.